In an 8 vestigial sideband (VSB) terrestrial communication system, information may be transmitted to a receiving station over the air. In an example of such a system, each two bits of data to be communicated is converted to a three-bit trellis code which is mapped as a symbol having eight possible levels so that each byte of data is represented by four symbols. The symbols are interleaved, and some of the symbols are rotated before they are transmitted in data frames.
A data frame that is used in an 8 VSB terrestrial communication system is disclosed in the Advanced Television Systems Comittee (ATSC) Digital Television Standard (published on Sep. 16, 1995) and particularly in Annex D, section 4.1 thereof. This data frame is comprised of a plurality of segments (i.e., lines) wherein each segment contains groups of data and wherein each group of data includes a plurality of data elements. For example, each segment may contain sixty-nine groups of data, each group of data may include twelve data elements, and a data element may be a symbol of data. The first segment of such a data frame contains frame synchronization information, and each subsequent segment in the data frame has an initial portion containing segment synchronization information (in the form of four segment sync symbols) and a succeeding portion containing data.
Because of the frame and segment synchronization information contained in a data frame, the data elements in certain groups of data may be rotated in order to facilitate the corrosion of transmission errors by the receiver. This rotation is described in Annex D, section 4.2.5 and in table 2 of the aforementioned Standard. Accordingly, the rotated and interleaved data received by a receiver must be de-rotated and de-interleaved.
The present invention is directed to an apparatus for de-rotating and/or de-interleaving data elements such as symbols.